The present invention is particularly, but not exclusively, concerned with a welding process used when laying pipes underwaiter, for example, when laying large diameter pipes with a pipe-laying vessel in the “S” laying configuration. Underwater pipelines are usually made up of many sections of pipe that are joined together by welding on a pipe laying vessel. The pipe-laying process is a costly and time consuming one. Each welded joint of a pipeline typically consists of several separate weld layers and the welding process is a significant contributor to the time taken to lay a pipeline. It is therefore desirable to reduce the time taken to weld a pipe section to the pipeline.
In a typical welding process, two adjoining pipes are welded together by a welding torch, or plurality of welding torches, such that a weld layer is created, the welding torch or torches are turned off and returned to their initial position, a further weld layer is created, and so on until the pipes are welded together as desired. The idling time, meaning the time for which the welding torches are turned off, can be as much as 20 to 30 seconds per lay cycle or even more. If a plurality of welding torches are used to build up the weld layers, for example a first welding torch creating a weld layer and a second welding torch following the first welding torch creating a second weld layer, a delay can be created by the time gap required to prevent the welding torches colliding with each other as they move around the pipes, or alternatively the time gap required to let the initial weld layer solidify sufficiently to allow a return pass to be made.
U.S. Pat. No. 4,145,593 discloses an automatic pipe welding system that includes a plurality of welding torches mounted on first and second torch transport assemblies, each torch transport assembly including two welding torches separated by 90 degrees around the circumference of the joint. In this case, the two pipes are horizontal, as is typical for a pipeline being laid in an S-lay configuration. The first and second torch transport assemblies are arranged such that each transport assembly welds one half of the joint from the top to the bottom of the joint. U.S. Pat. No. 4,145,593 states that the weld becomes more difficult as the welding torches depart from the 12 o'clock or upright positions and travel around the pipe joint. The welding torches weld a 90 degree sector of the joint and are then returned to their starting position as quickly as possible to begin the next welding sequence. There is therefore a delay before the next welding pass can be commenced, the length of the delay depending on how fast the torches can be returned to their starting positions.